This invention relates to the manufacture of polycrystalline diamond (PCD) materials.
Polycrystalline diamond, also known as a diamond abrasive compact, comprises a mass of diamond particles containing a substantial amount of direct diamond-to-diamond bonding. Polycrystalline diamond will generally have a second phase which contains a diamond catalyst/solvent such as cobalt, nickel, iron or an alloy containing one or more such metals.
When diamond particles are combined with a suitable metallic solvent/catalyst, this solvent/catalyst promotes diamond-to-diamond bonding between the diamond grains, resulting in an intergrown or sintered structure. This intergrown diamond structure therefore comprises original diamond grains as well as newly precipitated or re-grown diamond phase, which bridges these original grains. In the final sintered structure, solvent/catalyst material remains present within the interstices that exist between the sintered diamond grains. The sintered PCD has sufficient wear resistance and hardness for use in aggressive wear, cutting and drilling applications.
A well-known problem experienced with this type of PCD compact, however, is that the residual presence of solvent/catalyst material in the microstructural interstices has a detrimental effect on the performance of the compact at high temperatures. This decrease in performance under thermally demanding conditions is postulated to arise from two different behaviours of the metallic-diamond compact.
The first arises from differences between the thermal expansion characteristics of the interstitial solvent/catalyst and the sintered diamond network. At temperatures much greater than 400° C., the metallic component expands far more than the intergrown diamond network and can generate micro-fracturing of the diamond skeleton. This micro-fracturing significantly reduces the strength of the bonded diamond at increased temperatures.
Additionally, the solvent/catalyst metallic materials which facilitate diamond-to-diamond bonding under high-pressure, high-temperature sintering conditions can equally catalyse the reversion of diamond to graphite at increased temperatures and reduced pressure with obvious performance consequences. This particular effect is mostly observed at temperatures in excess of approximately 700° C.
As a result. PCD sintered in the presence of a metallic solvent/catalyst, notwithstanding its superior abrasion and strength characteristics, must be kept at temperatures below 700° C. This significantly limits the potential industrial applications for this material and the potential fabrication routes that can be used.
Potential solutions to this problem are well-known in the art.
One key approach is to remove the catalyst/solvent or binder phase from the PCD material, either in the bulk of the PCD layer or in a volume adjacent to the working surface of the PCD tool (where the working surface typically sees the highest temperatures in the application because of friction events).
U.S. Pat. Nos. 4,224,380 and 4,288,248 describe polycrystalline diamond compacts, initially sintered in the presence of metallic catalyst/solvents, where a substantial quantity of this catalyst/solvent phase has been leached from the diamond network. This leached product has been demonstrated to be more thermally stable than the unleached product.
There are several problems that result from this approach to achieving improved thermal stability. Firstly, these leached PCD pieces with their continuous network of empty pores possess a substantially increased surface area, which can result in increased vulnerability to oxidation (particularly at higher temperatures). This can then result in reduced strength of the PCD compact at high temperatures, albeit via a different mechanism. Porous leached PCD compacts of this type also suffer from technical attachment problems, in that they must still be brazed to a carbide substrate prior to use. Conventional PCD compacts are typically generated with the carbide substrate attached following the sintering step. This brazing step is technically challenging and often provides a subsequent weak point within the compact tool structure.
U.S. Pat. No. 4,944,772 discloses the formation of a bi-layered sintered PCD compact which has a top layer that is preferably thermally-stable. In one preferred embodiment, a leached PCD compact and a cemented carbide support are separately formed. An interlayer of unsintered diamond crystals (having a largest dimension of 30-500 μm) is placed between the carbide and thermally stable PCD (TSPCD) layer. A source of catalyst/sintering aid material is also provided in association with this layer of interposed crystals. This assembly is then subjected to HpHT conditions, sintering the interlayer and bonding the whole into a bi-layered supported compact. In this application, appreciable re-infiltration of the TSPCD layer is not seen as advantageous, but the requirement for some small degree of reinfiltration is recognised in order to achieve good bonding.
U.S. Pat. No. 5,127,923 teaches an improvement on this approach, where a porous thermally stable polycrystalline diamond (TSPCD) layer is reattached to a carbide substrate during a second HpHT cycle, with the provision of a second “inert” infiltrant source adjacent a surface of the TSPCD compact removed from the substrate. Infiltration of the TSPCD body with this second infiltrant prevents significant re-infiltration by the metallic binder of the carbide substrate. Where carefully chosen, it does not compromise the thermal stability of the previously leached body. A suitable infiltrant, such as silicon, for example, must have a melting point lower than that of the substrate binder.
It has been observed that compacts generated according to these teachings experiences high internal stresses because of the significant differences in properties between the leached/porous layer and the underlying sintered PCD and carbide substrate. This is exacerbated by the monolithic nature of the leached compact and often causes cracking at the PCD-substrate interface or through the PCD layer itself during the second attachment HpHT cycle. Furthermore, the reattachment process itself can be difficult to control such that appreciable re-infiltration of the TSPCD layer does not occur during the second HpHT cycle
Additionally, a further factor of concern is in the provision of the leached or porous TSPCD compact required. Typically, it is extremely difficult and time-consuming to effectively remove the bulk of the metallic binder from the finer-grained and thicker PCD tables required by current applications. In general, the current art is typically focussed on achieving PCD of high diamond density and commensurately PCD that has an extremely fine distribution of metal binder pools. This fine network resists penetration by the leaching agents, such that residual catalyst/solvent often remains behind in the leached compact, compromising its eventual thermal stability. Furthermore, achieving appreciable leaching depths can take so long as to be commercially unfeasible or require undesirable interventions, such as extreme acid treatments or the drilling of penetration channels into the bulk PCD, for example.
A further approach disclosed in the art pertains to the partial removal of the metallic binder from the PCD compact. JP 59219500 claims an improvement in the performance of PCD sintered materials after a chemical treatment of the working surface. This treatment dissolves and removes the catalyst/solvent matrix in an area immediately adjacent to the working surface. The invention is claimed to increase the thermal resistance of the PCD material in the region where the matrix has been removed without compromising the strength of the sintered diamond.
U.S. Pat. Nos. 6,544,308 and 6,562,462 disclose PCD cutting elements that are characterised inter alia by a region adjacent the cutting surface which is substantially free of catalysing material. The improvement of performance of these cutters is ascribed to an increase in wear resistance of the PCD in this area, where the removal of the catalyst material results in decreased thermal degradation of the PCD in the application.
Whilst substantial removal of the catalyst/solvent in this region to a depth of approximately 200-500 μm from the working surface does observably improve the performance of the cutting element in specific applications, certain problems are still experienced. As this approach is typically applied to a full cutting element i.e. with carbide substrate attached; the vulnerable substrate and PCD-substrate interface have to be masked or protected during the metal removal or leaching step. This masking process is not technically trivial and further limits the range of leaching treatments that can be employed without causing significant damage to the portions of the cutter that must be protected.
There is a further technical limitation inherent in this approach. The PCD layer is manufactured in situ on the carbide substrate and subsequently treated while attached thereto. Hence, the nature and type of the carbide substrate is restricted to that which is supportive of the infiltration and PCD sintering process. This restricts the optimisation of the mechanical properties of the substrate, to those which are coupled to suitable infiltration properties.